Method of treatment of steel parts in order to increase their resistance to wear and abrasion

ABSTRACT

A method of treatment of steel parts in order to increase their resistance to wear and abrasion, comprises essentially a mechanical operation of cold-deformation of the material effecting cold-working to a depth of about 1.5 mm. to 3 mm. and a complementary mechanical operation of cold-working at the surface, effected up to about 0.2 mm. to 0.5 mm. in depth, a gradient of residual stresses being obtained such that their value on the surface is higher than 80 daN/sq.mm. and decreases to about 30 daN/sq.mm. at a depth of about 0.4 mm. These operations may optionally be followed by an electro-chemical operation at low temperature, e.g., sulphuration.

United States Patent i1 1 Cros et al. Nov.-6, 1973 [54} METHOD OF TREATMENT OF STEEL 2,078,434 4/1937 Wise 72/366 PARTS IN ORDER TO INCREASE THEIR goffman omans...... RESISTANCE To WEAR AND ABRASION l47,502 2/1874 James 72/365 [75] Inventors: Georges Cros, Villars; Guy Mazet, 3,073,022 H 3 Bush et 72/53 saint chamond of France Caubet [73] Assignee: Centre Stephanois De Recherches Mecaniques Hydmmecanique Et Primary Examiner-Lowell A. Larson Frottement, Andrezieux-Boutheon, Thompson et France [22] Filed: Nov. 22, 1971 [57] ABSTRACT PP N05 200,965 A method of treatment of steel parts in order to increase their resistance to wear and abrasion, comprises [30] Foreign Application p i i Dam essentially a mechanical operation of cold-deformation NOV 23 1970 France 7041921 of the material effecting cold-working to a depth of 971 France 7140884 about 1.5 mm. to 3 mm. and a complementary mechanical operation of cold-working at the surface, effected 7 up to about 0.2 mm. to 0.5 mm in depth, a gradient of g 204/29 z l ggz residual stresses being obtained such that their value on [58] Fie'ld 366 53 the surface is higher than 80 daN/sq.mm. and decreases 5 2 to about 30 daN/sq.mm. at a depth of about 0.4 mm. These operations may optionally be followed by an [56] References Cited electro-chemical operation at low temperature, e.g.,

UNITED STATES PATENTS 5/1938 Erb 72/365 sulphuration.

9 Claims, 2 Drawing Figures PATENTEDHUV 605 3,770,595

SHEET 2 OF 2 METHOD OF TREATMENT OF STEEL PARTS IN ORDER TO INCREASE THEIR RESISTANCE TO WEAR AND ABRASION Various means have been suggested for the production of parts of steel which can readily be cold-worked. Amongst the known improvement methods there can be cited rollering, hammering, rolling and knurling, etc.

The range of conditioning generally adopted is as follows:

1. To effect a structural conversion at the surface of the parts.

2. To diffuse nitrogen, sulphur, etc., at a temperature in excess of 400C.

These various methods have three disadvantages:

1. Risk of adversely affecting the mechanical characteristics caused by a higher temperature of the treat ment of sulphuration.

2. The knurling operation results in a dimensional variation which is not the same from one part to the other. 3. The cold-working operation leaves on the surface of cause damage during running-in if they are set free by friction.

The present invention has for its object a method of treatment of steel parts in order to increase their resistance to wear and to abrasion, characterized by at least two mechanical operations of cold deformation, one a cold-working in depth, for example up to about 2 or 3 mm., the other to increase the cold-working at the surface, for example up to about 0.2 mm. to 0.5 mm. If so desired, these operations may be followed by an electro-chemical treatment in a bath of salts at low temperature.

The mechanical operations are such that on the one hand there is produced a gradient of residual stresses such that the value of these stresses at the surface is greater than 80 daN/sq.mm. and then decreases gradu ally so as to reach 30 daN/sq.mm. at about 0.4 mm. in depth, and such that on the other hand there is produced a cold-working gradient such that at at least 0.2 mm. in depth, the vickers hardness is greater than 550.

The electro-chemical operation is such that it leads to the formation at the surface of a superficial layer which inhibits welding, composed of a compound between a metalloid and the iron of the steel base, and such that this layer is obtained by immersion of the parts in a bath of salts at a temperature less than 400C., and preferably lower than 250C.

In one form of execution, the mechanical operations consist of a knurling and a rollering, and are followed by a sulphuration at low temperature.

The knurling operation has the object of coldworking the part to a depth of at least 1.5 mm., and. also to impress by forging on the surface of the part, patterns in a direction substantially perpendicular to the direction subsequently followed by the friction.

The rollering operation has the object, on the one hand of obtaining a surface vickers hardness greater than 700, and on the other hand to bring the part, which had increased in diameter after knurling, very exactly back to the dimension which it had before the first knurling operation, that is to say upon completion of machining.

The operation of sulphuration by electro-ch'emical means has the object, on the one hand of forming at the surface of the parts a layer of iron sulphide of about ten the metal fragmented zones or burrs which are liable to v microns, known as the weld-inhibitor layer, making it possible to prevent any risk of the part becoming welded during the running-in period on the other piece with which it is intended to co-operate in friction, and on the other hand to eliminate by chemical composition the burrs formed as a result of the first knurling operation. This sulphuration is carried out at very low temperature, namely below 400C., advantageously below 250C. and preferably below 200C., and does not in any way modify the characteristics of resilience of the parts. A sulphuration of this kind can be effected for example by electrolysis in a bath containing for the major part alkaline thiocyanates.

It will be appreciated that each of the three above operations individually makes it possible to improve the qualities of the part, the first by deep cold-working, the second by surface hardening, the third by the creation of a weld-inhibiting layer, and that furthermore there exist ties of co-operation between them having an effect of combination, in this sense that the second restores the limits and dimensions that the first has altered, while the third directly suppresses the burrs resulting from the first said operation and left by the second.

In an alternative form, the mechanical operations of cold deformation are effected by rolling and coiling.

In another alternative form, the mechanical operations of cold deformation are effected by rolling and hammering to shape.

In still another alternative form, the mechanical operations of cold deformation are effected by stamping and chasing.

Examples of embodiment of the method according to the invention are described below in a non-limitative sense, reference being made to the accompanying drawings, in which:

F161 is a diagram illustrating the gradient of residual stresses, in which there has been plotted as abscissae 0D the distance from the surface, or depth, in microns, and as ordinates OR the compression stress in daN/sq.mm.;

FIG.2 is a diagram illustrating the gradient of coldworking, in which there has been plotted as abscissae OD the distance from the surface or depth in microns, and as ordinates 0V, the vickers hardness HV under a load of grams.

EXAMPLE 1 There are treated a shaft and a ring of 40 mm. in diameter which are made from a HADFIELD steel containing 1.2 percent of carbon, 12 percent of manganese, in the supertempered state at l050C. with a powerful cooling.

1. These parts are first subjected to a knurling operation under the following conditions:

The knurling tools carry two 'knurling wheels of 20 mm. in diameter, producing on the parts two families of striations, the depth of which is comprised between 0.3 mm. and 1.0 mm. and preferably in the vicinity of 0.7 mm. The striations are spaced apart from each other by a distance comprised between 1 mm. and 2 mm. and preferably in the vicinity of l mm. The striations are transverse with respect to the direction specified for the friction and are advantageously inclined with respect to this direction. This inclination is preferably 60, which amounts to saying that the striations are inclined by 30 to the direction of the generator lines;

Each knurling wheel passes 20 times in front of the same point of the part to be cold-worked;

The force of application of the knurled wheels (length of generator line of the wheel: 10 mm.) on the part is comprised between 200 kg and 1000 kg and preferably in the vicinity of 600 kg.

The diameter of the parts thus conditioned is increased by 0.7 mm. 2. These parts are then subjected to a rollering operation under the following conditions:

The tool carries two smooth wheels, that is to say without striations and cambered with a radius of camber comprised between 100 mm. and 300 mm., and preferably in the vicinity of 200 mm.;

The force of application of the wheels on the part is comprised between 100 kg and 5 00 kg and preferably in the vicinity of 300 kg;

Each wheel passes 20 times opposite a same point of the part.

After this second operation, the shafts and rings have been restored to the dimensions which they had before the first operation, namely after completion of machining, that is:

40 H7 for the shaft;

40.1 H8 for the ring.

After these first two operations, the shaft and ring parts of HADFIELD steel show in cross-section, under metallographic examination, extremely dense systems of slip lines penetrating to a depth of more than 1 mm. The micro-hardness indicates on the one hand hardnesses greater than 700 HV for the first three-tenths of a millimetre from the surface, and on the other hand a progressive fall in this hardness beyond 0.3 mm., the hardness of the base metal (250 HV) being found only at a depth of between 1 mm. and 1.5 mm.

3. The parts are then subjected to an electro-chemical sulphuration at a low temperature, less than 250C.

This sulphuration is effected in an electrolytic bath containing for the major part alkaline thio-cyanates, the temperature of which is advantageously comprised between 150 and 200C. for a duration comprised between 2 and 30 minutes and preferably in the vicinity of 10 minutes. The anode is formed by each part treated and the anode current has a density comprised between 1A and 10 A per sq.dm. and preferably in the vicinity of 3 A per sq.dm.

More particularly, the sulphuration treatment has the following characteristics:

Cathode: crucible of ferritic stainless steel;

Composition of the bath in percentage by weight:

Potassium thio-cyanate 66 percent Sodium thio-cyanate 32 percent Potassium ferricyanide 2 percent Temperature of the bath: l90C.;

Density of anode current: 3.5 A/sq.dm.;

Duration: 10 minutes.

After micrographic examination there is found:

No loss of hardness during the course of the sulphuration treatment;

The disappearance of the burrs created during the cold-working operation;

The presence on the surface of a layer of iron sulphide comprised between 2 and 20 microns and preferably in the vicinity of 10 microns, and very exactly equal to 8 microns in thickness, known as the weld-inhibitor layer, making it possible to avoid risk of the part becoming welded to the counterpart with which it is intended to co-operate in friction during the first hours of operation;

No fall in the resilience.

Parts thus conditioned are capable of greatly increased performances under friction as compared with those which could be obtained with parts of the same grade but which were not subjected to the three operations referred to above.

In the example described above, the shaft of 40 mm. in diameter is allowed to oscillate in a ring of 40 mm. in length under a pressure of 200 bars.

The assembly is mounted with very slight lubrication and with an initial play of 0.1 mm. The amplitude of the oscillations is 90 and their frequency is l Hertz.

The shaft and the ring treated according to the present example of embodiment have oscillated for 1450 hours. When they were dismantled, the wear on the shaft was 0.2 mm. and on the ring 0.3 mm.

If, all other things remaining equal, the shaft and the ring were subjected only to a simple operation of knurling, at the end of only 500 hours working, much greater wear is found: wear on the shaft 0.5 mm. and wear on the ring 0.7 mm.

It will be appreciated that one of the mechanical operations of cold deformation effects cold-working in depth, for example up to 2 mm. in the example shown in FlG.2 (this could be more, for example 2.5 mm. or 3 mm., or less, for example 1.5 mm.) while the other mechanical operation completes the cold working on the surface, for example and without any limitation, up to about 0.2 mm. or 0.5 mm.

The operations are such that the gradient of the residual stresses corresponds to a curve comprised between the curves C and C of P101, and that the gradient of cold working corresponds to a curve comprised between the curves C and C of P162.

It will be noted from FIG.1 that the residual stress gradient is such that the value of these stresses at the surface is greater than daN/sq.mm. and then gradually decreases so as to reach 30 daN/sq.mm. at about 0.4 mm. of depth. It will be observed from FIG.2 that the cold-working gradient is such that below at least 0.2 mm. in depth, the hardness is greater than 550 HV.

It will be appreciated that the electro-chemical operation leads to the formation on the surface of a surface layer which inhibits welding, made of a compound between a metalloid and the iron of the steel base, and that this layer is obtained by immersion of the parts in a bath of salts at a temperature less than 400C, and preferably less than 250C.

EXAMPLE II The procedure is the same as in Example I, but with the third operation of sulphuration modified as follows:

Cathode: crucible of ferritic stainless steel; Composition of the bath in percentage by weight:

Potassium thio-cyanate 80 percent Sodium thio-cyanate 10 percent Sodium cyanide 4 percent Sodium sulphide 3 percent Alkaline carbonate 3 percent Temperature of the bath: 200C.; Density of anode current: 3 A/sq.dm. Duration: 6 minutes.

The performances obtained are similar to those of Example 1.

EXAMPLE III This concerns a method of treatment of a tape, comprising a succession in the order given, of the three following operations:

a. A rolling operation for the purpose of coldworking the material;

b. A coiling operation having the purpose on the one hand of increasing the cold-working by deformation (drawing, bending) and on the other hand of obtaining the desired geometrical shape;

c. Optionally but preferably, an electro-chernical operation in a bath of molten salts forming a weldinhibiting layer at the surface of the parts.

The whole of these two operations of rolling and coiling may be performed in a non-limitative example by means of a machine on which the tape unwinds between two jaws effecting the cold-working of the material by rolling, and is then wound on a cylindrical mandrel permitting an increase in the cold-working and Shaping.

The two mechanical operations of rolling and laminating are such that the characteristics described in Example l are obtained (FIGS.1 and 2) while the electrochemical operation may also be carried out as described in the said Example 1.

EXAMPLE IV This is concerned with a method of treatment of a ribbon, comprising the succession, in the order given, of three operations:

a. An operation of rolling of a ribbon for the purpose of obtaining the desired geometrical shape;

b. An operation of hammering to shape, the purpose of which is on the one hand to cold-work the surface and thedepth of the metal by deformation, and on the other hand to obtain a well-defined geometry, while respecting close tolerances;

c. Optionally, an electro-chemical operation in a bath of molten salts forming a weld-inhibiting layer at the surface of the parts.

The two mechanical operations of rolling and hammer-shaping are such that the characteristics described in Example 1 are obtained (see FIGSJ and 2) while the electro-chemical operation can also be effected as described in the said Example 1.

EXAMPLE V This relates to a method of treatment of a metal sheet, comprising the succession, in the order stated, of the following operations:

a. A stamping-chasing operation on the metal sheet in a deforming machine. This operation makes it possible on the one hand to obtain cold-working at the surface and in depth by deformation of the material, and on the other hand to preserve a definite 1 geometric shape while respecting close tolerances.

' This method may be applied for example by means of a deforming machine on which is mounted a mandrel, the material to be deformed being imprisoned in this mandrel and by a plate-clamp. A mechanical or hydraulic system makes it possible to chase the plate to be deformed and to give it'the desired geometrical shape.

b. Optionally, an electro-chemical operation in a bath of molten salts.

The two mechanical operations of stamping and chasing are such that the characteristics described in Example I (H681 and 2) are obtained, while the electrochemical operation may also be carried out as described irf Example I.

It will be appreciated that the methods which have been described above make it possible to obtain con- 10 jointly a set of mechanical and chemical characteristics which give the steel its optimum resistance to wear and to abrasion. These characteristics are as follows:

Establishment of a gradient of residual stresses (FIG.1) such that the values of these stresses at the surface are higher than daN/sq.mm. and then progressively decrease so as to reach 30 daN/sq.mm. at about 0.4 mm. in depth. A gradient of residual stresses of this kind may be obtained either by allotropic surface conversion of the metal (change of phase 1 a), or by cold-working properly so-called. This gradient of values of residual stresses makes it possible to avoid surface destruction of the parts by over-stresses of tension arising behind the zone of contact of the two parts in frictional contact.

Establishment of a cold-working gradient (FlG.2) such that under a depth of at least 0.2 mm., the hardness is greater than 550 HV. It is especially important that the hardness of the base metal should only be found below a depth of 1.5 to 2 mm.

Production of a weld-inhibitor layer at the surface of at least one of the frictional parts. This weldinhibiting layer, which must on the other hand he obtained at a low temperature (less than 400C. and preferably less than 250C may be a sulphide or a telluride or an iodide of iron, etc., and its function is particularly beneficial in the case of parts working under large stresses and operating under conditions of doubtful lubrication.

The production of parts complying with the different criteria such as previously defined, thus permits an improvement to be obtained in performances under friction by pushing back the threshold of damage due:

to wear by abrasion (increase of hardness at the surface and in depth);

to seizure (by diffusion of weld-inhibiting elements);

to brutal tearing accidents by over-stresses of ten- SlOn.

It will of course be understood, as indicated in the foregoing description, that still higher performances of frictional parts may be obtained by producing at the surface of at least one of the two parts co-operating in frictional contact, striations substantially perpendicular to the direction of movement.

Non-limitative' tests of the method according to the invention are described below. These tests were carried out on a simulator utilizing a shaft of 50 mm. in diameter oscillating in a ring of 50 mm. in diameter and 50 mm. in length, under a pressure of 100 bars, the whole unit being mounted with slight lubrication and with an initial play of 0.1 mm. The amplitude of the oscillations was and their frequency was l Hertz.

1. First test sample Shaft and ring of steel with 1.2 percent of carbon and 12 percent of manganese, previously super-tempered and having undergone no treatment either mechanical or chemical (total absence of cold-working, stresses and weld-inhibitor layer). There occurred an almost instantaneous seizure resulting in blocking of the articulation at the end of a few hours working (generally between 2 and 5 hours). 2. Second test sample Shaft and ring of steel with 1.2 percent of carbon and 12 percent of manganese, previously super-tenpered and then heat-treated in order to increase the mechanical characteristics by partial decomposition of the austenite. In this case, the seizure phenomenon generally occurs after 20 hours working. It should also be noted that parts thus treated have their resilience considerably reduced to a value close to zero. 3. Third test sample Shaft and ring of steel with 1.2 percent of carbon and 12 percent of magnesium, previously super-tempered and then slightly cold-worked by a single knurling operation. Under these conditions, no pair has been able to exceed 300 hours of operation. From the first moments of operation there is observed a fairly large amount of wear, both of the shaft and of the ring. 4. Fourth test according to the present invention The shaft is of steel with 1.2 percent of carbon and 12 percent of manganese previously super-tempered and then knurled, rollered and treated by sulphuration in an electro-chemical bath of salts at a temperature lower than 400C. and preferably in the vicinity of 200C. This method makes it possible to obtain compression stresses at the aurface equal to 1 l0 daN/sq.mm. and a surface hardness of 850 HV.

The ring is produced by rolling and then winding of a steel ribbon with 1.2 percent of carbon and 12 percent of manganese previously super-tempered. There are thus obtained residual compression stresses at the surface equal to 90 daN/sq.mm. and a surface hardness of 750 HV. These tests were stopped after 1450 hours working. No sign of seizure could be observed. The wear on the shaft and on the ring was of the order of 2/10 mm.

What we claim is:

1. A method of treatment of steel parts in order to increase their resistance to wear and abrasion, said method comprising cold deforming the material effecting cold-working to a depth of about 1.5-3 mm, and further cold-working the surface of said material to a depth of about 0.2-0.5 mm.

2. A method of treatment as claimed in claim I, in which a gradient of residual stresses is obtained, such that the value of said stresses on the surface is higher than daN/sq.mm. and then progressively decreases so as to reach 30 daN/sq.mm. at a depth of about 0.4

3. A method of treatment as claimed in claim 1, in which a cold-working gradient is obtained such that at a depth of at least 0.2 mm. the hardness is higher than 550 HV.

4. A method of treatment as claimed in claim 1, in which said mechanical operations of cold deformation are carried out by knurling and roller-ing.

5. A method of treatment as claimed in claim 1, in which said mechanical operations of cold-deformation are carried out by rolling and coiling.

6. A method of treatment as claimed in claim 1, in which said mechanical operations of cold-deformation are carried out by rolling and swaging.

7. A method of treatment as claimed in claim 1, in which said mechanical operations of cold-deformation are carried out by stamping and chasing.

8. A method of treatment as claimed in claim I, and thereafter forming on the surface of the material a superficial layer composed of a sulphur-containing compound between a metalloid and the iron of the steel base.

9. A method as claimed in claim 1, and thereafter removing burrs from the surface of the material by forming on the surface of the material a superficial layer composed of a sulphur-containing compound between a metalloid and the iron of the steel base. 

2. A method of treatment as claimed in claim 1, in which a gradient of residual stresses is obtained, such that the value of said stresses on the surface is higher than 80 daN/sq.mm. and then progressively decreases so as to reach 30 daN/sq.mm. at a depth of about 0.4 mm.
 3. A method of treatment as claimed in claim 1, in which a cold-working gradient is obTained such that at a depth of at least 0.2 mm. the hardness is higher than 550 HV.
 4. A method of treatment as claimed in claim 1, in which said mechanical operations of cold deformation are carried out by knurling and rollering.
 5. A method of treatment as claimed in claim 1, in which said mechanical operations of cold-deformation are carried out by rolling and coiling.
 6. A method of treatment as claimed in claim 1, in which said mechanical operations of cold-deformation are carried out by rolling and swaging.
 7. A method of treatment as claimed in claim 1, in which said mechanical operations of cold-deformation are carried out by stamping and chasing.
 8. A method of treatment as claimed in claim 1, and thereafter forming on the surface of the material a superficial layer composed of a sulphur-containing compound between a metalloid and the iron of the steel base.
 9. A method as claimed in claim 1, and thereafter removing burrs from the surface of the material by forming on the surface of the material a superficial layer composed of a sulphur-containing compound between a metalloid and the iron of the steel base. 